Are There Limits to Superlubricity of Graphene in Hard, Rough Contacts?

Abstract

Yes, there are. They result from the splitting of a large correlated contact into many small patches. When the lubricant consists of thin solid sheets, like graphene, the patches are expected to act independently from each other. Crude estimates for the friction forces between hard, stiff solids with randomly rough surfaces are given, which apply to surfaces with Hurst roughness exponents H > 0.5. The estimates are obtained by combining realistic contact-patch-size distributions with friction-load relations deduced for isolated contact patches. The analysis reveals that load is carried predominantly by large patches, while most frictional forces stem from small contact patches. Low friction is favored when the root-mean-square height gradients are small, while a large roll-off wavelength and thus large root-mean-square roughness is predicted to lead to small friction. Moreover, friction is found to increase sublinearly with load in a nominally flat, structurally lubric contact.

Sliding Friction of Amorphous Asperities on Crystalline Substrates: Scaling with Contact Radius and Substrate Thickness

“…If a is larger than λ, the lateral forces between surfaces no longer cancel and τ saturates at a constant value for large a . Even for smaller a , analogous arguments predict pinning of macroscopic surfaces by deformation at scales larger than the asperity spacing. ,, …”

“…There has been great interest in friction of thin solid layers of graphene or MoS 2 as realizations of nearly two-dimensional (2D) behavior and as practical replacements for liquid lubricants. ,,,− We show that the scaling of λ and τ/τ 0 is different for amorphous asperities on such thin systems and that friction decreases as film thickness h increases. Experiments have observed a similar trend for free-standing films of different h , and we discuss how our results relate to past explanations. − …”

“…Even for smaller a, analogous arguments predict pinning of macroscopic surfaces by deformation at scales larger than the asperity spacing. 15,16,31 For crystal surfaces, λ is related to the core size b core of a dislocation that accommodates the mismatch between the periods of the surfaces, allowing them to lock in local registry. 22−26 Simulations for a range of systems 25,26 shear stress between identical aligned crystals because parts of the interface could advance independently.…”

Structural lubricity of physisorbed gold clusters on graphite and its breakdown: Role of boundary conditions and contact lines

“…In fact, Özoğul et al (2017) found superlubric states for metal clusters adsorbed on graphite despite conducting their experiments under ambient conditions. Roughness can also lead to pinning, for example, through many small contact patches that carry relatively little normal load but, due to their small size, exert relatively large frictional shear stresses [ Müser (2019) ]. Finally, when interfacial interactions dominate the ones inside the bulk, which happens when chemical bonds form across the interface [ Dietzel et al (2017) ], two solids or clusters have no choice but to pin.…”

Structural lubricity of physisorbed gold clusters on graphite and its breakdown: Role of boundary conditions and contact lines

“…In fact, Özoǧul et al [32] found superlubric states for metal clusters adsorbed on graphite despite conducting their experiments under ambient conditions. Roughness can also lead to pinning, e.g., through many small contact patches that carry relatively little normal load but, due to their small size, exert relatively large frictional shear stresses [28]. Finally, when interfacial interactions dominate the ones inside the bulk, which happens when chemical bonds form across the interface [8], two solids or clusters have no choice but to pin.…”